A major study has concluded that there are no major genetic causes of six common diseases, including diabetes and multiple sclerosis.
The British researchers say patients suffer from a "complex combination" of hundreds of small, weak changes in genes – with each change being found widely in the population.
The researchers at Queen Mary, University of London, say the findings suggest the diseases result from combinations of genetic and environmental factors.
They say there is no evidence that any of the diseases are caused by small numbers of rare genetic mutations.
The researchers set out to study six autoimmune diseases – thought to be caused by the immune system going awry and attacking the body.
These were coeliac disease, Crohn’s disease, multiple sclerosis, the skin disease psoriasis, type 1 diabetes and autoimmune thyroid disease.
Their findings were reported in the journal Nature.
The researchers compared some 25,000 people with these diseases with another 17,000 people free from them, studying variants in some 25 genes previously linked to the diseases.
Researcher Professor David van Heel said: "These results suggests that risk for these autoimmune diseases is not due to a few high-risk genetic variations but seems rather due to a random selection from many common genetic variants which each have a weak effect.
"For each disease there are probably hundreds such variants and the genetic risk is likely to come from inheriting a large number of these variants from both parents."
He added: "If this is the case then it may never be possible to accurately predict an individual’s genetic risk of these common autoimmune diseases."
One of the researchers was Professor Richard Trembath, vice-principal of the Barts and The London School of Medicine.
He said: "The results prompt a re-assessment of the genetic architecture that determines risk for development of common auto-immune disorders and will fuel future careful assessment of regions of the human genome beyond those presently known to confer susceptibility to these important medical conditions."
Nature 22 May 2013
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